1.Field of the Invention
This invention relates, in general, to coated abrasive material. More particularly, it relates to novel coated abrasive material suitable for use in a single step fining pad in ophthalmic applications.
2. Technical Background and Prior Art
The term "fining" is an established term in the ophthalmic art. A description of the fining process and of suitable machinery for accomplishing it are disclosed in U.S. Pat. No. 3,732,647 (to Stith) and U.S. Pat. No. 4,320,599 (to Hill et al), the complete specifications and drawings of which are herein incorporated by reference. Stith discloses in FIG. 2 of the patent, a lapping tool such as envisioned by one aspect of the instant invention. The lapping surface 78 of the tool provided in Stith may be a coated abrasive material consisting of abrasive grains adhered to a flexible backing which, in turn, is supported by the structure disclosed in Stith.
When "ophthalmic lens fining" is performed on a Coburn-505 fining machine with coated abrasive material, however, it can refer to either a "one-step" process or a "two-step" operation. In one-step (or single step) fining, a single daisy wheel or film backed fining pad ("Snowflake") is employed before the final slurry-polishing step. Such a pad is capable of removing relatively large amounts (0.4-0.6 mm) of excess stock and, at the same time, generate a sufficiently fine, scratch-free surface. In the more conventional two-step fining operation, a silicon carbide (600 grit) coated abrasive product (a first fining pad) is used first which removes most of the surplus stock. This is followed then by use of a second fining pad, a much finer grain aluminum oxide based, coated abrasive product. This second pad removes little stock (0.03-0.05 mm) but has fine finishing capabilities. Preference for the one-step or the two-step process heretofor has depended upon a number of factors, including the lens type to be ground (glass, CR-39 plastic and polycarbonate are the three most common lens types), the lense curvature (diopter), shape (cylindrical and spherical), and lens size.
Nevertheless, whether the one- or two- step process is used, the main objective of lens fining is to prepare the optical lens for the final or slurry polishing step which is usually performed with slurries of various small particle size aluminum oxide (0.5-1.0 micron range). As a consequence of such low particle size, the slurries cannot remove deep scratches (Rt values greater than, say, 50-70 microns) from lenses obtained during the fining process. Thus, there is always a need for fining products that better improve the results of the fining or prefinishing process so as to reduce the burden, both in time and in fine polishing requirements, placed on the final slurry-polishing step.
Heretofore, in U.S. Pat. No. 4,644,703, which was issued on Feb. 24, 1987 to Wesley R. Kaczmarek, Eugene Zador, and Sitaramaiah Ravipati, and which is assigned to Norton Company, the Assignee of the instant application, there has been disclosed coated abrasive material suitable for use in a single step lens fining process. The product disclosed in that patent is manufactured by coating two distinct layers of an adhesive/abrasive grain slurry onto a backing member, to provide a coarse outer layer and a finer inner layer of abrasive grains. The slurry coatings are deposited by a gravure roll having a trihelical pattern cut therein which, in turn, imparts a pattern of parallel lines of adhesive/abrasive grain slurry to the backing member and, in turn, to the first deposited coating. Subsequent to application of the first coating, the backing member with the wet slurry thereon passes through a texturing bar assembly whereat the continuity of the deposited coating material, i.e., the lines of wet slurry, is broken up to provide a somewhat discontinuous pattern. Afterwards, the wet slurry coating is subjected to ultraviolet light to cure the adhesive binder and to adhere the abrasive grains to the backing member. After curing of this first slurry coating, a second adhesive/abrasive grain slurry is coated onto the first coated backing member, to provide the outer, more coarse grain, layer in the coated abrasive product. This processing is the same except that a gravure roll having a different helical pattern is used, and there is no texturing of the second applied wet slurry.
The abrasive grains, in U.S. Pat. No. 4,644,703, are adhered to the backing member, which may be a polyester film, with binder layers compounded primarily of two distinct groups of ingredients as the main components in somewhat different formulations for the respective first and second coats. The radiation curable binders, in general, comprise a mix of monomers of different functionality and an acrylated oligomer. Thus, the binders are formulated with a carefully selected combination of monomers including triacrylated monomers, e.g., trimethylolpropane triacrylate (TMPTA), and diacrylated monomers, e.g., hexanediol diacrylate (HDODA), and acrylated oligomers, the preferred being the diacrylates of epoxy resins of the bisphenol-A type, to obtain the desired combination of hardness, low degree of shrinkage, curing speed and adhesion. Importantly, also, as disclosed by the patentees, the adhesive formulations include a non-acrylic monomer, i.e., N-vinyl-2 pyrrolidone in a controlled amount. Such a monomer, among other things, promotes adhesion to the substrate, and serves as a viscosity reducer for the slurry.
In any event, the single step two-layered fining pad of U.S. Pat. No. 4,644,703 has met with only somewhat limited commercial success. Such an abrasive product must meet certain desired cut parameters and the cut performance of the two-layered fining pad has been found to be on the lower end of the required range.
Recently, there has been commercially introduced by others a further coated abrasive product for use as a single step fining pad. This product has what appears to be spherical-shaped aggregate of aluminum oxide abrasive particles (4 microns) coated onto a backing member. In use, the product is claimed to provide a high initial cut rate which is maintained because new abrasive is exposed as the aggregates wear down. Further, such product is claimed to fine a lens to a dimensionally precise surface with a pre-polish surface previously unattainable in a single fine operation. Nevertheless, although good performance may be obtained with this product, as claimed, its use is not without certain disadvantages. First, its method of manufacture necessarily incurs added cost in the use of the aggregates. These aggregates must, of course, be first manufactured, after which they are then coated onto the water-proof paper backing member. The resin used for coating of the spherical-shaped aggregates onto the backing member is a phenolic, i.e., phenol-formaldehyde. The use of such binders in and of themselves involve certain problems and disadvantages, e.g., long curing times. Importantly also is the fact that such resin binders contribute to environmental problems giving off, during curing, toxic fumes of phenol and formaldehyde. Furthermore, in use, the performance does not always seem consistent in producing the desired finish. In some cases, numerous deep scratches on the lens have resulted in use of this product.
Thus, there still remains a need for an improved single step fining pad. And, there is further a need for a system comprising a radiation curable adhesive binder which will allow manufacture of such ophthalmic coated abrasive products of good quality and in a relatively low-cost commercially satisfactory manner.